1. Field of the Invention
The present invention relates to lead-free optical glasses with indices of refraction nd between 1.49 and 1.55 and with Abbxc3xa9{acute over ( )} numbers xcexdd between 47 and 59.
2. Description of the Related Art
Since the glass components PbO and As2O3 have entered public discussion as being environmental pollutants, the manufacturers of optical instruments demand PbO-free and preferably also As2O3-free glasses having appropriate optical properties.
Simple replacement of the lead oxide by one or more ingredients is generally not a successful way of reproducing the desired optical and glass performance properties affected by PhO. Instead, new developments or substantial modifications to the glass composition are necessary.
Lead-free optical glasses having optical values in the above range and similar compositions are already known. However these glasses have various disadvantages.
DE 196 09 735 A1 describes crown flint glasses which have a high SiO2 content and therefore are relatively difficult to melt and have high melting temperatures.
U.S. Pat. No. 3,940,278 describes a glass for optical glass fibers. Although its rather low SiO2 content and rather high Al2O3 content are a way to achieve the desired flow behavior at high temperatures and, chemical resistance, it is necessary to use very large amounts of Na2O and BaO in the glass. Because of these high levels of Na2O and BaO, these glasses become very susceptible to crystallization.
EP 0 645 349 A1 describes an optical glass consisting of the system SiO2xe2x80x94Nb2O5xe2x80x94R2Oxe2x80x94F and further optional components. Because of its Nb2O5 level of up to 15% by weight, the glass is very susceptible to crystallization.
It is an object of the present invention to provide a lead-free optical glass having a refractive index nd between 1.49 and 1.55 and an Abbxc3xa9 number xcexdd between 47 and 59, which is inexpensive to produce, has good melting and processing properties and sufficient crystallization stability.
According to the invention the lead-free optical glass has an index of refraction (nd) between 1.49 and 1.55 and an Abbxc3xa9{acute over ( )} number (xcexdd) between 47 and 59, and comprises (in percent by weight on an oxide basis):
and optionally at least one fining agent in an amount sufficient for fining.
The glasses contain the glass-forming oxides SiO2 (60-70 percent by weight) and Al2O3 (0.3 to 5 percent by weight), SiO2 being the main glass former. Preference is given to a minimum SiO2 content of  greater than 60 percent by weight. If the above-mentioned maximum Al2O3 content were exceeded, the devitrification tendency and the melting temperature would increase excessively. Preference is given to a maximum Al2O3 content of  less than 5% by weight. On the other hand, omission of this second glass former would lead to a reduction in chemical resistance. In total, the high glass former level of at least 60.3%, by weight and up to 75% by weight results in a relatively high viscosity.
This is counteracted by the high level of Na2O (16-25 percent by weight) which acts as fluxing agent for improving the meltability. In addition, the glass formers, in this ratio to the fluxing agent Na2O, have a beneficial influence on the glass xe2x80x9clengthxe2x80x9d so that these glasses are easy to process. At even higher levels, in particular in the presence of TiO2 and ZrO2, the crystallization tendency would be increased excessively. For the same reason, Li2O is completely omitted.
The glasses can contain both TiO2 (up to 9 percent by weight) and ZrO2 (up to 7 percent by weight). These two components improve the chemical resistance, but do not reduce the meltability as much as Al2O3. At higher levels, the crystallization stability would be substantially reduced, as with Al2O3. In addition, higher TiO2 levels, together with the iron ion impurities in the glass, promote yellow discoloration of the glasses by formation of ilmenite.
The crystallization tendency is counteracted by the addition of both components, because in this case the potential pure crystal structure is disturbed, and therefore it is possible to incorporate higher total levels of both components than of a single component. In this way, it is also possible to counteract ilmenite formation and thus discoloration of the glasses. For these reasons, compositions, which contain both ingredients, are preferred in terms of crystallization stability.
Both components are also used in order to achieve the desired refractive index and dispersion range. Both components make it possible to establish a high refractive index at the same time as a low Abbxc3xa9 number. Here, it is likewise preferred to use both components in order to facilitate establishing a specific optical status by variation.
For this reason, the glasses according to the invention can additionally contain up to 7% by weight of Ta2O5 and/or up to  less than 0.5% by weight of Nb2O5. Higher levels of Nb2O5 would increase the crystallization tendency of the glasses. In particularly preferred embodiments, the glasses are free from Nb2O5. At the above-mentioned levels, these components allow the optical status to be varied in particularly wide ranges with the same base glass composition. These components are in particular used in order to achieve medium Abbxc3xa9 numbers at particularly high refractive indices. Furthermore it is possible to achieve X-ray opacity in the glasses by using Ta2O5, just as by using TiO2 or ZrO2, at higher levels within the claimed range. The glasses preferably contain at least two percent by weight of a sum total of TiO2+ZrO2+Ta2O5. It is thus already possible to obtain X-ray-opaque glass bodies from these glasses. In order to achieve adequate X-ray opacity, even at low thickness of the glass bodies, the glasses should contain a total of at least 4.5 percent by weight of these components. A further increase of their content would, in certain combinations of the three components determining the optical status, lead to nd and vd deviations from the desired values and to an unnecessary increase in batch price owing to the unneccesarily high Ta2O5 levels. Furthermore, adequate potential X-ray opacity is already achieved with the claimed amounts.
It is therefore preferred that the sum total of TiO2, ZrO2 and Ta2O5 is no more than 15 percent by weight. Glasses having a TiO2+ZrO2+Ta2O5 content of between 2 and 15 percent by weight have refractive indices nd between 1.50 and 1.55 and Abbxc3xa9 numbers xcexdd between 47 and 57.
For precise adjustment and thus to counterbalance the components adjusting the low Abbxc3xa9 numbers at the same time as high refractive indices, the glasses can contain up to 3 percent by weight of F. In addition, low amounts of F increase transmission by suppressing ilmenite formation (complex formation with FeIII) and can have an additional fining effect, so that these glasses have significantly fewer bubble defects than comparable glasses containing no F.
In order to improve the glass quality, one or more fining agents known per se can be added to the batch in conventional amounts in order to refine the glass. The glass then has a particularly good internal quality with respect to freedom from bubbles and streaks.
If the fining agent used is not As2O3, but instead, for example, Sb2O3, which is possible without losses regarding the glass quality, the glasses, which are lead-free according to the invention, are in addition free from arsenic, except for trace arsenic oxide impurities. The Sb2O3 content is preferably between 0.1 and 0.5 percent by weight.
Within the above-mentioned composition range, there are various groups of particularly preferred composition ranges.
On the one hand, in particularly preferred embodiments the glass comprises (in % by weight, based on oxide): SiO2 less than 60-70, preferably 63-70; Al2O3, 0.3- less than 5; Na2O, 16-25, preferably 19-24; TiO2, 0 to 9; ZrO2, 0-7; Nb2O5, 0- less than 0.5, but preferably Nb2O5-free; Ta2O5, 0-7, wherein a sum total of TiO2+ZrO2+Nb2O5+Ta2O5, 4.5-15, or preferably a sum of TiO2+ZrO2+Ta2O5, 4.5-15.
These glasses have refractive indices nd between 1.50 and 1.55 and Abbxc3xa9 numbers xcexdd of between 50 and 57, and, with compositions from the preferred ranges given, refractive indices nd between 1.50 and 1.53 and Abbxc3xa9 numbers xcexdd between 51 and 57. These glasses are X-ray-opaque.
On the other hand, the following glasses (in percent by weight, based on oxide) are particularly preferred, because they exhibit a particular optical position at the same time as high crystallization tendency: SiO2 less than 60-70, preferably 63-70; Al2O3, 3- less than 5; Na2O, 16-25, preferably 19-24; TiO2, 3-9; ZrO2, 0.1-2; Nb2O5, 0 less than 0.5, preferably 0-0.1; F, 0-3.
The relatively high TiO2 content serves to adjust the refractive index.
The glasses have refractive indices nbetween 1.51 and 1.55 and Abbxc3xa9 numbers xcexdd of between 47 and 54, and, with compositions from the preferred ranges, refractive indices nd between 1.52 and 1.55 and Abbxc3xa9 numbers xcexdd between 47 and 53.
Especially preferred embodiments of the invention include glasses of the following composition (in percent by weight, based on oxide): SiO2, 63-70; Al2O3, 114  less than 5; Na2O, 19-24; TiO2, 3-6; ZrO2, 0.1-2; which have refractive indices nd between 1.51 and 1.54 and Abbxc3xa9 numbers xcexdd between 50 and 53, and glasses of the following composition (in percent by weight, based on oxide): SiO2, 63-70; Al2O3, 0.3-1; Na2O, 19-24; TiO2, 6-9; ZrO2, 0.1-2; and F, 0.5-3; which have refractive indices nd between 1.52 and 1.55 and Abbxc3xa9 numbers xcexdd between 47 and 50.
Both groups of glasses are Nb2O5-free and therefore have a particularly high crystallization stability. The former glasses containing more Al2O3 and less TiO2 differ from the latter glasses containing less Al2O3, more TiO2 and F by a significantly higher Abbxc3xa9 number with equally good melting behavior.
The glasses according to invention have the following advantages in addition to the desired optical properties:
The glasses are PbO-free and, in a preferred embodiment, also As2O3-free. The glasses have good crystallization stability. This enables production in a continuous melting unit. A measure of crystallization stability, which is adequate for a product of this type, is the viscosity at the upper devitrification limit. For continuous production, it should be xe2x89xa71000 dPas. This is the case with the glasses according to the invention. Crystallization stability of the glasses also enables further thermal treatment of the glasses, such as pressing or re-pressing. A very good processing range is also ensured by the length of the glasses.
The glasses not only have good processing properties, but also good melting properties. This is also evident from their melting points of about 1330xc2x0 C.
The glasses have excellent chemical resistance, evident from their classification in alkali resistance class AR 1 (ISO 10629) and in acid resistance class SR 1 (ISO 8424). These resistances in each case may be 1.x. The chemical resistance of the glasses is of importance for their further treatment, such as grinding and polishing.